1. Field of the Invention
The present invention relates to a position sensing device which senses the position of a moving object such as a drive ring provided in a camera, for example, to obtain optical data such as a focal length and the full-open aperture value of a lens.
2. Description of the Related Art
Recently, many cameras equipped with a power zoom lens have been developed. In a camera to which such a zoom lens is fitted, usually the full-open aperture value is changed in accordance with a focal length of the lens, and accordingly, to carry out an automatic exposure control in such a camera, the full-open aperture value must be sensed and input to the camera body. Further, in a camera in which an exposure program, for example, is changed in accordance with a focal length of the lens, data regarding the focal length of the lens must be input to the camera body.
Therefore, in a conventional device, the position of a zoom ring (a drive ring), which causes the zoom lens groups to move close to and apart from each other, is sensed and optical data such the a corresponding full-open aperture value and the focal length of the lens is obtained from the lens position data.
The movement of the zoom ring is sensed by counting the number of changes of a pattern, via a counter, in which periodically varied codes are given to the entire range of movement of the zoom ring.
FIG. 2 shows a plan view of an example of a pattern of a conventional device. As shown in the drawing, in this pattern, a first element A absorbing or cutting off a beam and a second element B reflecting or transmitting a beam, each having a same length and size are alternately and repetitously arranged on and move integrally with the zoom ring. Accordingly, if a photo reflector and a photo interrupter are provided, to apply a beam to the pattern and receive a reflected or transmitted beam, therefrom the amount of light received from the first element A is low, and the amount of light received from the second element B is high. Thus, the position of the zoom ring is sensed by counting the number of changes in the amount of light received, from a base position.
As described above, since the conventional device senses only the number of changes of the amount of light reflected by the pattern, from the base position, a base switch, such as microswitch must be provided at the base position, to determine the base position. Therefore, not only is the construction thereof complicated, but the device is also bulky and expensive.
Further, due to a poor contact with or corrosion of the base switch, reading errors may occur.